Plants and animals never live alone. They are always interacting in some form or another with other living things. All types of interaction affect the environment by making it easier or more difficult for some species to survive. There are many different types of interactions. The most obvious is the interaction between predator and prey- where the predator is trying to eat the prey species while the prey is trying to run away. Living things also interact by competition - they struggle against each other to get enough resources. There is another form of interaction that is often less apparent. It's called symbiosis.

Symbiosis is any type of intimate long-term interaction between two different types of organisms. In symbiotic relationships, the two organisms aren't trying to kill each other and they aren't trying to fight over food, but they still have a significant impact on each other.

There are three types of symbiosis: parasitism, mutualism, and commensalism. In a parasitic relationship, one organism is harmed while the other benefits. It's a relationship that is very one sided. A common example of a parasitic relationship that we should all be painfully aware of is the relationship between mosquitoes and mammals, like humans. The mosquito needs to suck the blood from animals in order to lay its eggs. Without the blood, its eggs would die and no more mosquitoes could be born. The animal that is bitten is harmed because it loses some blood and has to put up with a painful mosquito bite.

A more drastic example of parasites are tapeworms that spend most of their lives in the gut of their hosts. The tapeworm steals nutrients from the food in its host's intestine; without a host, the tapeworm is unable to live. The host is harmed by the tapeworm because much of its food that it eats is used by the worm. The tapeworm rarely causes its host to die, but the host suffers from weight loss and decreased energy, as well as many other health problems.

Mutualism, on the other hand, is a relationship in which both organisms benefit. A classic example of a mutualistic relationship is the combination of algae and fungi, called lichens. Lichens are often mistaken for a type of moss. The algae part of the lichen use the fungi as a place to live. The fungi protect the algae from the environment and keeps it from drying out. The fungi benefits by being able to use the sugar the algae makes through photosynthesis. The algae can't live without the fungi, and the fungi can't live without the algae.

A more familiar example of mutualism is the relationship between fruiting plants and animals that eat fruits. All plants work to make sure that their seeds get dispersed so that the parent plant isn't competing with its offspring for sunlight and water. Fruiting plants have solved this problem by covering their seed with a tasty fruit. Animals come along, eat the fruit, and walk away. Most of the seeds inside the fruit pass through the animals digestive tract unharmed some distance from the parent plant. The animal benefits by getting to eat the tasty and nutritious fruits and the plants benefit by getting its seeds dispersed. Of course many of the seeds won't survive and the plant had to produce fruit in the process, but no one ever said that both organisms had to benefit equally.

Commensalism is a symbiotic relationship where one organism benefits and the other is neither hurt nor harmed. A good example many of us are probably familiar with deals with plants, like burdock, that disperse their seeds by making them sticky. Animals walk by the plant and the seeds stick to the animal. The seeds ride around on the animal until either the seed loses its stickiness or the animal picks it off. This way, the plant gets to easily disperse its seeds and the animal is not harmed. Many times, the animal might not even notice the seeds.